This invention relates to a method for significantly reducing the levels of natural resins which are present in wood. The method described herein is characterized by the significant economic benefits it provides, without compromising the quality of the wood pulp produced.
When harvested, trees contain varying concentrations of compounds known in the art generically as resins. These compounds include fatty acids, rosin acids, sterols, hydrocarbons and fat. Although present in relatively small quantities, these resins create significant processing difficulties in manufacturing products from processed wood pulp. The problems are particularly acute in the case of hardwood derived alpha cellulose which is used in manufacturing viscose rayon, cellophane, plastics, and nitrocellulose. Resins are present in lower overall concentrations (on the order of 0.1 to 2%) in most species of hardwoods than are found in softwoods, but their reduction to acceptable levels for processing is more difficult.
Methods have been described previously for reducing the levels of resin in wood during processing. The emphasis on chemical methods of deresination has been placed on certain nonionic surfactants, such as certain condensation products of ethylene oxide. For example, U.S. Pat. No. 2,716,058 describes a class of deresination agents which includes ethoxylated organic substances including phenols and natural products such as tall oil. According to the patent, these agents are preferably used in the caustic extraction stage of the bleaching process. Other chemical agents are described, for example in U.S. Pat. Nos. 2,999,045 (copolymers of polyethylene oxide and polypropylene oxide) and 3,446,700 (condensation products of vicinal epoxide and phenol). U.S. Pat. No. 2,144,756 describes a two-step process for removing "pitch" including the addition of an organic solvent, such as kerosene, to dissolve the pitch and thereafter adding an emulsifier, of which sulfonated fatty acids are one suggested class, to aid in removing the pitch in a subsequent stage of processing.
The chemical effectiveness of existing deresination methods is not the sole consideration for a manufacturer of wood pulps, and particularly producers of alpha cellulose. Instead, the degree of desired deresination must be balanced against the cost of the methods utilized, and particularly the cost of the chemical deresinators. In accordance with the present invention, a deresinating composition is employed which comprises a mixture of an effective high-cost deresinator with a less expensive deresinator or extender, which composition achieves a comparable degree of deresination to that achievable with a full concentration of the more expensive deresinating agent.
Owing to the complexities of the deresinating mechanism and the mystique associated with the process, the process is not entirely understood. The deresinator may function as both a solubilizer for the resin, while at the same time serving as an emulsifier for the resin during its removal. Mixtures of deresinating agents may or may not function desirably since often one chemical in the aqueous media will alter the solvent or emulsifying properties of another. For example, U.S. Pat. No. 4,426,254 suggests that improvements in the solubility of a deresinating agent in an alkaline medium through the addition of a C.sub.12 alpha-olefin sulfonate is desirable. Although enhanced solubility in the medium may be desirable, enhanced solubility is by no means synonymous with enhanced effectiveness as a deresinator. Improvements of the deresinator's solubility in the water phase (cooking liquor) arguably could deleteriously affect the emulsification of the resin due to the change in the hydrophilic-hydrophobic balance (HLB) of the deresinator blend. Thus, the resin extraction functionality of the deresinator, in fact, may be reduced.
It becomes readily apparent that the availability of an economical, effective deresination process is highly desirable. Accordingly, an object of the present invention is to provide a highly effective method of deresination which enables a commercial user to reduce its costs.